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Both of these statements are true, and understanding how that can be the case is critical to becoming a good ETF trader.

The bid/ask spread

The place to start with understanding how ETFs trade is to understand how individual stocks trade.

At any given time, there are 2 prices for any common stock: the price at which someone is willing to buy that stock (the “bid”) and the price at which someone is willing to sell (the “ask”). The difference between these 2 prices is called the “spread.”

The reason spreads exist is because, in any open market, folks try their best to negotiate the best prices they can get. If you’re looking to buy, you’ll naturally want to see if someone is willing to sell for less than the last traded price.

Conversely, if you’re selling, you’ll naturally hope that someone will bend and be willing to buy it for more than the last quoted price. Spreads are simply the result of buyers and sellers negotiating on prices.

For example, let’s imagine Microsoft’s stock is trading with the bid at $49.90 and the offer at $50.10. The spread is therefore $0.20. If someone asked you what a share of MSFT was “worth,” you would probably choose the midpoint, $50.00, or maybe the last price at which you can see a trade actually happened.

But if you wanted to buy MSFT right now, you would probably have to pay $50.10. If you wanted to sell right now, all you’d get is $49.90. Those are the prices you’d get if you enter a market order into your brokerage window.

The wider the spread, the more it will cost you to trade MSFT.

Bid/ask spreads are so important to ETF trading because, unlike a mutual fund, which you buy and sell at net asset value, all ETFs trade like single stocks, so ETFs trade with bid/ask spreads. That’s the price of the “exchange-traded” in the name.

Spreads widen and narrow for various reasons. If the ETF is popular and trades with robust volume, then bid/ask spreads tend to be narrower. But if the ETF is thinly traded, or if the underlying securities of the fund are highly illiquid, that can also lead to wider spreads.

Overall, the narrower the bid/ask spread, the lower the cost to trade.

Volume and market impact

Bid/ask spreads aren’t the only factor to consider when trading, whether you’re trading stocks or ETFs. You also have to look at volume and so-called market impact.

Volume is the number of shares that trade on any given day. The higher the volume, the better. For example, if MSFT trades, on average, 10 million shares per day, it will be easier to trade than something that trades 100 shares per day. Note, however, that spreads could be tight on both, which could mislead unwitting investors to conclude that both securities are equally liquid.

Typically, the number of shares offered on the bid or the ask will be small—sometimes 100 shares, sometime more, but rarely a huge amount. If you try to buy 10,000 shares of something that only trades 100 shares per day, you could have trouble.

To go back to our MSFT example, someone might be willing to sell you 100 shares of MSFT at $50.10, but if you want to buy 10,000 shares, you might have to pay $50.25 or more. The amount that you drive up the price of something you are trying to buy is called the market impact.

How does that impact ETF trading, and how are ETFs different?

Because ETFs trade on exchanges like stocks, they have bid/ask spreads, volumes, and potential market impact, too. All else equal, you will do better trading something that has high volume and a tight bid/ask spread. In this way, trading ETFs is just like trading a stock.

But ETFs have a critical difference that dramatically alters the playing field for investors.

With single stocks, there is no way to create new shares. If someone wants to buy 10,000 shares of MSFT, he or she must find another investor who wants to sell. If no one wants to sell, he or she may have to pay a lot of money to get that trade done.

But ETFs are different: A group of institutional investors called authorized participants (APs) are allowed to create new shares of an ETF to meet demand. So if you want to buy a lot of an ETF—say 50,000 shares—an AP might create those shares to fill your order.

Next steps to consider

Article copyright 2014 by ETF.com. Reprinted with permission from ETF.com. The statements and opinions expressed in this article are those of the author. Fidelity Investments cannot guarantee the accuracy or completeness of any statements or data.

Exchange-traded products (ETPs) are subject to market volatility and the risks of their underlying securities, which may include the risks associated with investing in smaller companies, foreign securities, commodities, and fixed income investments. Foreign securities are subject to interest rate, currency exchange rate, economic, and political risks, all of which are magnified in emerging markets. ETPs that target a small universe of securities, such as a specific region or market sector, are generally subject to greater market volatility, as well as to the specific risks associated with that sector, region, or other focus. ETPs that use derivatives, leverage, or complex investment strategies are subject to additional risks. The return of an index ETP is usually different from that of the index it tracks because of fees, expenses, and tracking error. An ETP may trade at a premium or discount to its net asset value (NAV) (or indicative value in the case of exchange-traded notes). The degree of liquidity can vary significantly from one ETP to another and losses may be magnified if no liquid market exists for the ETP's shares when attempting to sell them. Each ETP has a unique risk profile, detailed in its prospectus, offering circular, or similar material, which should be considered carefully when making investment decisions.

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ETFs are subject to market fluctuation and the risks of their underlying investments. ETFs are subject to management fees and other expenses. Unlike mutual funds, ETF shares are bought and sold at market price, which may be higher or lower than their NAV, and are not individually redeemed from the fund.

Important legal information about the e-mail you will be sending. By using this service, you agree to input your real e-mail address and only send it to people you know. It is a violation of law in some jurisdictions to falsely identify yourself in an e-mail. All information you provide will be used by Fidelity solely for the purpose of sending the e-mail on your behalf.The subject line of the e-mail you send will be "Fidelity.com: "